Hydrometallurgical process for the recovery of tin



May 25, 1943- J. R. s TAcK 2,319,887

HYDROMETALLURGICAL PROCESS FOR THE RECOVERY OF TIN Filed June 28, 1940 Patented May 25,n 1943 HYDBOMETALLURGICAL PnocEss Fon 'rim naoovnnr or 'rnv James n. sck, Pittsburgh, Pa'.; Aivuas L. stack, Montpelier, Vt., executrix of said James yIt.

Stack, deceased ApplicatlonJune Z8, 1940, Serial No. 342,957-

This invention relates to a process for the recovery of tin from ores or concentrates and more particularly to a process for therecovery of tin from ores or concentrates which are normally insoluble such, for example, as stannic oxide (SnOz) or other tin containing substances which may or may not contain iron as an impurity.

On account of the refractory nature and insolubilty of tin oxide, it has been the psual prac` tice in the art to reduce the tin oxide in the concentrates by smelting. This reaction is highly endothermic and a unit reverberatory furnace charge requires about twelve hours at Aapproximately 13007 C. Such a process is bothtime consuming and expensive and for its economic` operation requires a pure high grade tin concenf trate. Therefore, a hydrometallurgical process in which the tin oxide in lower gradeconcentrates is converted at a lower temperature and shorter time into a compound from which the agents; (4)` subjecting the puriiled solution to electrolysis to recover the dissolvedtin, using an linsoluble anode of the porous diaphragm typ and purifying the acid for reuse.

Tin, iron and certain other metals are amphoteric and stannates, ferrates, etc., are formed when oxides of these metals are heated with the oxide, hydroxide, or carbonate of an alkaline earth metal, particularly calcium, barium' or strontium. The latter compounds are character` ized by being much more soluble in certain acids than the oxdies of tin, iron, etc., from which they t. metals are readily available.

tin can be recoveredf'by leaching and electrolysis .is very desirable. l

It is accordingly an object of the pesent invention to provide a process suited to the recov eryiof. tin from sources which have heretofore been considered unprofltable, such, for example, as low grade ferruginous tin concentrates.

Another object is to provide a novel and im provedl process for the efliciet and economical recovery of tin from 'various sources.

Another object is to provide a novel and imwere formed. is particularly true of tin, The alkaline earth metals are easily eliminated by precipitation in subsequent operations.

The oxides, hydroxide, and carbonates of these The cheapest of these reagents to use is calcium carbonate in the form of limestone. The product then formed, in case of tin, is calcium-stannate (CaSnOz) and in case of iron, calcium ferrate (CaFeOs) and perhaps some calcium ferrite (CaFesO:)'. These products are readily soluble in certain acids, for example, hydrochloric acid, sulfuric acid hydrouor'ic acid, fiuosllicic acid, fluoboric abid and certain organic acids or mixtures of acids.

The temperature at which the calcining is carried out may vary from 950 C. to 1400 C. though somewhat higher temperatures are not harmful. The time of calcination ofcourse varies with the character of the tinconcentrates and the tem# proved leaching processfor" recovering tin fr m sources of the above type.

A further object is to provideV a novel and improved method for electrodepositing tin.

Otherobjectsand advantages will be apparent as the nature of the invention is more fully disclosed.

Although thesnovel features which are believed to be characteristic o! this invention are more particularly pointed out in the claim appended hereto, the invention itself -may be better understood by referring to the following description in which a specliic embodiment thereof has been set forth for purposes of illustration.

'I'he a ow sheet illustrating, for example, an embodiment of the invention using hydrochloric acid forthe lixiviant.

The present process in general comprises five steps-(l) calcining th'e nely divided 4tin con centrates with a cheap chemical compound to form a new compound which readily soluble in certain acids; (2) leaching thejcalclne in suitable acids to dissolve the tin; (3) purifying the solu- A m tion by cementation'with metallic tin or :other 65 rundes may interfere witlr'some or the subse'.

perature used. In general, for a given tin conpcentrate the lower` the temperature the longer the time required for thesame degree of conversion. In normal operations I have found that substantially complete conversione: the tin oxide to the soluble stannate form can be accomplished by calcining at a temperature of 1300* C. for a duration of approximately one hour, in any suitable manner such as by` the batch method with or without rabbling, or ina continuous process ln or Herreschoi type furnace.

In one embodiment, the iinely dividedtin ore or concentrates, s preferably to 200 mesh, are

Lintimately mixed with powdered linie rockl (cacoa), using a slight excesssr the stpicniometric proportions required to combine with the tin, iron andim ities to form calcium stannate,

calcium ferrate. tc. If the tin is not present in amechanical hearthroaster, suchas a Wedge It is advantageous to use ``asource of lime quent steps such as the acid regeneration, to be described.

The mixing maybe performed in a ball mill or While various acids capable of dissolving calcium stannate and calcium ferrate may be used, I have factory.

The residue may be separated by settling, decantation or filtration and thorough washing. It should contain the dore and substantially all of the gangue, such as silica, alumina, etc. The residue may be further worked by conventional methods for recovery of gold and silver. The lixiviant contains substantially all of the tin, iron.

' found hydrochloric acid to be particularly satisand other metallic impurities together with a minor portion of the silica. Y

The lixiviant may be treated by agitating the hot solution in the presence of a suitable reducing agent, such as metallic tin plates, to cement out the easily reducibl metallic impurities such as lead, copper, arsenic, antimony, bismuth. etc., and to reduce the stannic tin and ferric iron to the stannous form and ferrous form respectively, from which condition the tin can be deposited. While the reduction and cementaticn can be effected by metallic tin, other suitable metals, such as iron or nickel or chemical reducing agents may be used either singly or in combination with metallic reduction.

The cemented impurities may be separated by settling and decantation and the solution filtered to remove any suspended solids. Thev separated impurities may be :hlter pressed or centrifuged and washed and may be sold to smelters for the metal values such as bismuth, copper, etc.

The purified solution contains essentially Sn++, Ca++ and H+Cl and may be electrolyzed with insoluble anodes using standard multiple system tanks for the electrolyticcells and electrolytic diaphragms for the anode compartments to prevent the alternate oxidation and reduction of tm at the anode and cathode respectively, which would interfere with the eiilciency of depositing tin at the cathode.

The anode proper may consist of a lead electrode surrounded by a porous diaphragm using a dilute solution of sulfuricacid as the anolyte; The puried tin rich solution from which tin is to be recovered may constitute the catholyte. In order to prevent mechanical mixing of the anolyte and catholyte the porous diaphragms may be impregnated with silica Jell. While tin is deposited at the cathode, oxygen is liberated at the anode and may be recovered, if desired.

The deposit from a tin chloride solution is very difficult to control even with the use of :the so-called addition agents and frequently results in a dendritic, needle-like, crystalline deposit. which is non-adherent and tends to cause short circuits. However, I have folmd that. by the controlled addition ofa soluble copper compound inthe presenceof glue, thus codepositing a fracof tin can be obtained. The copper and glue may be added separately or asa mixture.

The tin may be deposited on tin starting sheets or on -tinned sheet iron. The cathode tin may then be melted and decopperized according to standard practice in tin, lead, and solder refining, for example, by stirring the molten metal in a kettle until a vortex is formed and adding sulphur, which combines with the copper and comes to the surface as a dry dross or matte contaminated with some tin. The copper dross or matte may be removed and roasted to remove sulfur and then leached with acid to dissolve the copper, and the residue of stannic oxide may be returned to the calciner -for retreatment or the copper (tin) matte may be sold to smelters. 'I'he remaining tin is of a'high degree of purity and constitutes the final product of the process.

The tin free hydrochloric acid solution after electrolysis'contains calcium and iron as the major impurities which must be eliminated to` regenerate the acid.

The calcium may be precipitated almost quantitatively by adding the stoichiometric amount or an excess of sulfuric acid, and the calcium sulfate precipitate may be separated from the solu-- tion by thickening and may be washed free of acid by countercurrent decantation in a series of thickeners. Y,

The iron may be separated from the hydrochloric acid in various ways-one method is by distillation. using the .waste heat from the calcining furnace and absorbing the hydrogen chloride gas in water to form hydrochloric acid according toitandard practice in the manufacture of the ac l The above description refers to batch operation. When operating as a continuous process present in solution and may be removed if de# siredVv by special chemical methods. However, the quantities would be small since their occurrence in tin concentrates is rare.

Inthe above described process hydrochloric acid is used as a leaching agent because of its relative cheapness. and availability. h It is to be understood, of course, that other leaching agents capable of dissolving the' stannate which is formed during the calciningstep may be emploed. Examples of such agents are sulphuric tion of a per cent of copper, anadherent deposit 76 i hydroiiuoric acid, iluoboric acid and iiuosilicio acid, certain organic .acids or mixture of acids. When other acids than hydrochloric are usedfrom which tin can be deposited in adherent metal radical will be subsequently removed from; the electrolyte by rsteps similar to thoseabove described. Calcium has been referred to in the speciiic example only as an illustration.

The present process is particularly applicable to the recovery of tin from relatively low gradetin ores and concentrates in which the tin is oi comparatively low concentration and is accompanied by various impurities. In Bolivian concentrates, for example, the impurities may oo'm-- prise (1) easily reducible materials, such as lead, copper, arsenic, antimony and bismuth; (2) more diiiicultly reducible materials, such as iron; (3)

gangue or slag -forming materials, such as silica, alumina, lime, tungsten and titanium and (4) Y for depending upon thev particular acid used.

matte forming materials, such as sulphur. Usu-V Y ally these impure materials require soxne form 'of treatment before smelting. However, the

present process is suited to the direct recovery of `the tin values from. such raw materials both eiiieiently and economically-and results in a. minimum amount of waste. No fume, slag, hard head (a tin-iron alloy) or troublesome by-products are produced. In the calcining step the tin oxide in the concentrates is converted into an acid soluble compound which may be readily dissolved by leaching.

It is to be understood that the tin may be separated from the lixiviant by various chemical methods, if desired, such, for example, as ,by

- oxidation, aeration, crystallization and precipitation. 'me partimllr ffm`f ecm t setltorth herein. however. his bee! rmmdtobeandtomaterlallyreduce theoostoitinrecovery, y

While a specific embodiment of the invention has been set forthior purposes oi illustration, it is to be understood that various changes and modiiications may be made therein as will be readily apparent to a person skilled in the art. 'lhe invention is only to be limited in accordance with the scope of the following claim.

What is -claimedisz N The process for recovering tin from a hydrochloric acid bath containing tin in solution which comprises electrolyalngthe bath using insoluble anodes ot the porousdiaphragm type while adding a soluble copper compound and glue to deposit the tin cathodically together with a-small amount, less than" 1% of copper, sepa rating the cathodic tin from the copper and recovering thetin.

. JAMES R BTAUK. 

